The present invention relates to monitoring the velocity of a jet of a papermaking machine along a slice opening and for controlling the jet velocity.
Conventional papermaking machines have a head box which contains a slurry of liquid and paper forming fibers, along with other additives, used in the formation of web or sheet products. For purposes of this description, and without limiting the generality of the invention, these products will be collectively referred to as paper, with this term being broadly construed to encompass any product made by depositing a slurry of liquid and fibers into a sheet or web. Papermaking machines have an elongated slice opening through which the slurry is deposited onto a moving wire or screen which assists in separating the fiber from the liquid, inasmuch as the liquid passes through the screen. This web or mat of fibers is then typically passed through a dryer for drying the product into the finished paper. Actuators are conventionally used for adjusting the slice opening to permit the passage of greater or lesser amounts of slurry from the slice opening to, for example, change the basis weight or quality and grade of the paper being produced. Both computer and manually controlled actuators have been used. These actuators are typically positioned at various locations along the slice opening for adjusting the slice opening at these locations as required. Due to manufacturing tolerance and other variations, the actuators are not necessarily set at the same position as they are used to adjust the slice opening to compensate for these factors.
In one conventional approach, the basis weight of the paper is tested following drying by the dryer. In the event the basis weight varies transversely across the width of the paper, the various actuators are adjusted in an attempt to adjust the slice opening to bring the basis weight to a more uniform level. However, many paper machine operators are reluctant to allow the adjustment of slice opening actuators during the manufacture of a particular grade of paper because of the risk of further reducing the quality of the resulting product. In addition, by sensing the basis weight downstream from a dryer, adjustments are less quickly accomplished in response to a variation in paper quality. U.S. Pat. No. 4,374,703 of Lebeau et al. is one example of a control system for a papermaking machine head box having a basis weight sensor downstream from a drying section.
The effect of jet velocity on the quality of the resulting paper has been previously recognized. In U.S. Pat. No. 4,086,130 of Justus, pressure transducers are positioned in a head box for use in monitoring the velocity of the slurry or stock flowing through channels of a multi-channel paper machine. The total head at the slice opening is regulated in response to the pressure transducer signals to maintain a desired spouting or jet velocity. The use of pressure transducers for the indirect measurement of jet velocity does not account for variations in jet velocity along the length of a slice opening. In addition, the adjustment of the head to control velocity can be a relatively slow reacting velocity control approach.
In a prior art previously marketed system sold by Beloit Corporation, an optical sensor was used for monitoring the jet velocity at a single location along the slice of a papermaking machine. To function, the optical sensor was positioned to nearly contact the jet. Because of the adverse environmental conditions at this location, purge water was directed toward the optical sensor for cleaning purposes.
Another example of a prior art papermaking machine control system is disclosed in European Published Patent Application No. 82108478.7 to Helleur. This patent application describes slice actuators which are heated to expand and move the edge of an adjoining portion of a slice opening to control the size of the opening. A magnetic field is used to heat the actuators. Adjustment of the slice opening for purposes of controlling caliper, basis weight, opacity, show-through, and other properties related to the uniformity of web structure is mentioned. This patent also mentions tying of devices for sensing these properties to a closed-loop system to control the slice orifice and thus the desired property of the final web. Without providing any explanation, this patent also mentions that devices could also measure the components or other properties issuing from the slice, e.g., dry/wet substance, jet velocity, etc.
There is some discussion in the literature of the use of microwave doppler-effect flow monitors for monitoring the surface velocity of fluids. For example, an article entitled "Interrogating Flow Fields With Radar and Laser Sources" by Harris, Volume 3, No. 11, 1970, in Measurement and Control, is one example. An article entitled "Microwave Doppler-Effect Flow Monitor" by Hamid, published in Volume IECI-22, 1975, of the IEEE Transactions On Industrial Electrical and Control Instrumentation is another example. However, these references do not suggest the use of microwave velocity sensors in the harsh environments associated with the papermaking machines and in particular where signals from a microwave sensor would be expected to vary as a result of variations in reflections from the paper forming slurry. Variations in signal reflections can arise, for example, when the grade or basis weight of paper is changed and from turbulence in the jet at the slice opening.
The use of microwave-doppler-effect velocity measuring devices in other applications is also known. For example, U.S. Pat. No. 4,633,252 to Bachman et al. describes such a mechanism for measuring the velocity of a vehicle. In the specific embodiment described in this patent, preamplifier and automatic gain control circuitry are employed.
Therefore, a need exists for an apparatus and method for more effectively monitoring the velocity of a jet at the slice opening of a papermaking machine and for controlling jet velocity. In addition, a particular need exists for a device for monitoring and controlling jet velocity at plural locations along a slice opening.